Bare, as well as patterned, semiconductor wafers are checked for the presence of defects, particularly for the presence of particles. Prior art methods and devices are used in order to detect foreign substances in patterned wafers.
One method comprises scanning the wafer surface with a laser beam and analyzing the number and direction of diffraction lights, produced by the pattern edges, by means of a plurality of light detection cells arranged cylindrically. Other methods involve using polarized light, or comparing an inspected pattern with an ideal pattern, or analyzing the light reflected from a wafer surface by distinguishing between normal directions and abnormal directions due to reflection from particles, or employing a planar array of individually addressable light valves for use as a spatial filter in an imaged Fourier plane of a diffraction pattern.
U.S. Pat. No. 4,731,855 includes in its Background of the Invention a list of various methods for performing semiconductor wafer inspections, and said list is incorporated herein by reference.
The methods and apparatus of the prior art have several drawbacks, partly discussed in the cited references, such as errors due to faulty registration and other causes, false alarms consisting in the detection of defects that are only apparent, and so on. All of them, further, have the common defect of requiring complex apparatus, with high mechanical precision, and requiring long operation times and having therefore a low throughput.
In order to overcome the disadvantages of the prior art, the applicant has invented a method and an apparatus that are described and claimed in a copending patent application, Ser. No. 09/110,870 filed concurrently with this application, the contents of which are entirely incorporated herein by reference. The prior art methods do not permit the identification of particles having extremely small dimensions, particularly smaller than the width of the pattern lines, e.g. sub-micron particles. Such identification is difficult even with the method and apparatus of the aforesaid copending application. Modern wafers have pattern widths in the order of 0.18-0.5 .mu.m. Particles having smaller sizes, e.g. a size of about 0.1-0.2 .mu.m, are particularly difficult to detect. Their detection is difficult on uniform surfaces, such as those of unpatterned wafers or memory areas of patterned wafers, as well.
It is therefore a purpose of this invention to permit the detection of such particles.
It is another a purpose of this invention to permit the detection of such particles in each pixel of the controlled surface, without reference to its pattern and without comparing patterns.
It is a further purpose of this invention to provide a high speed detection method which has a very high throughput and permits the on-line detection of sub-micron particles.
It is a still further purpose of this invention to provide a detection method that is compatible with the method that forms an object of the aforesaid copending application and can be carried out concurrently with it.
It is a still further purpose of this invention to provide an apparatus for the detection of sub-micron particles.
It is a still further purpose of this invention to provide an apparatus that can be combined with the apparatus that forms an object of the aforesaid copending application, to form a single unit.
Other purposes and advantages of the invention will appear as the description proceeds.